Most file systems use hierarchical tree structure of directories to catalog data stored in storage media. Directories of a file system store references to all the files (and sometimes other directories) residing on the file system volume.
In a typical hierarchical tree structure of directories in a file system, a single directory stores references to all files/directories residing under that directory. The layout of a directory indicates how that directory stores information about files residing under it. Any access to files first start by looking up the file in the hierarchical tree structure starting from the file system's “root” directory or in a specific directory. The lookup operation typically involves disk accesses to go through the directory layout to search for the file. The performance efficiency of the lookup depends on the kind of layout the directory employs. As the number of files in a directory increases, the time needed to lookup a file typically also increases.